Dispatch signals for more resource allocation events

[knikolla]

As a developer that is developing plugins for ColdFront, I would like my plugin's code to be executed in realtime (or close to) when specific resource allocation events are performed.

This behavior can be implemented by registering methods in the plugin as receivers for particular Django signals.

ColdFront already dispatches signals for adding and removing a user from a resource allocation, and uses this pattern in the FreeIPA plugin. However, the exposed signals are insufficient for a complete implementation of a generic resource allocation plugin, ex. for OpenStack.

For that, additional signals are required to be dispatched for:

• creation of a new resource allocation (Added signal dispatch for resource allocations #319)
• removal/disabling of a resource allocation (Added signal dispatch for resource allocations #319)
• changing of resource allocation attributes